In many applications it is desirable for gas and/or electrical charge to diffuse or otherwise pass into or through a solid layer. Often, however, the solid material may not support useful rates of diffusion or other mechanisms for passing gas and/or charge. By way of example, catalyst particles dispersed in a so-called catalyst layer in an electrochemical cell or fuel cell provide a catalyzing medium spread over an area. In order for the catalyst particles in the layer to be active, they will desirably have relatively easy access to gas and be able to conduct electrical charge. The layer may also be required to conduct protons. Proton conducting materials, however, often do not provide good support for gas and/or charge diffusion and conduction.
Catalyst layers are of particular utility when used with electrochemical cells employing a membrane and electrode assembly, with an example being proton exchange membrane (PEM) fuel cells. These cells may include a membrane electrode assembly (MEA) consisting of a proton exchange or solid polymer electrolyte membrane coated on one or both of its operative surfaces with a catalyst layer. Fuel and air are converted to electricity on the MEA, with the catalyst layer providing for higher conversion rates. The active catalyst typically comprises precious metal particles, and most commonly one or more of Pt, Pd, or Ir; or Pt or Pd alloyed with one or more of Pd, Ru, Mo, Ni, Fe, Co, Mn.
Because of their expense, it is desirable to minimize the amount of catalyst particles required while maintaining high levels of cell efficiency. Accordingly, it is desirable for high proportions of the catalyst particles to be active. Also, providing thin catalyst layers may be useful to minimize the catalyst particle cost. Catalyst inks that include catalyst particles suspended in a solvent are useful for providing a thin catalyst layer. For example, it is known to spread a catalyst ink on a proton exchange membrane and then hot press the resultant structure to remove the solvent and fix the catalyst particles near the surface of the membrane. Such a method is described in detail, for example, in U.S. Pat. No. 5,234,277 to Wilson et al., and “A NAFION-Bound Platinized Carbon Electrode For Oxygen Reduction In Solid Polymer Electrolyte Cells,” A. K. Shukla et al., J. Appl. Electrochem., 19 pp. 383–386 (1989). These methods and resultant catalyst layers, however, have problems associated with them. For example, hot pressing can result in damage or destruction of the membrane, and can result in poor mass transfer and proton conductivity within the catalyst layer. These problems cause reduced fuel cell efficiency and current output from the cell.
A solution to some of the problems of these prior art inks and layers was proposed in U.S. Pat. No. 5,415,888 to Bane jee et al. Generally, the '888 patent teaches the addition of proton-conducting polymers, such as NAFION (NAFION is a registered trademark of the DuPont Chemical Corporations, Wilmington, Del.), to a catalyst ink. Inclusion of the proton-conducting polymer reduced the need for hot pressing and its related disadvantages. In practice, the catalyst ink is applied to the proton exchange membrane and the solvent is removed, leaving catalyst particles embedded in a thin layer of the cast proton-conducting polymer. In addition to acting as an electrolyte to provide a path for proton conduction away from the catalyst, the solid proton-conducting polymer also fixedly holds the catalyst particles in position.
The teachings of the '888 patent leave several problems unresolved. For example, a portion of the active catalyst is often buried in the proton-conducting polymer layer, which generally does not support good rates of gas diffusion or electron conduction. In a buried state the catalyst particle is thus essentially inactive and wasted because it is not easily accessible to reactants (i.e. fuel or air/oxygen) or to the current collector. Wasted catalyst keeps the relative cost of these methods and layers high and lowers their efficiency. Also, the cast form of the proton-conducting polymer that results when the catalyst ink dries typically does not have satisfactorily, high proton conductivity. These and other problems are discussed in “Effects Of NAFION Impregnation On Performances Of PEMFC Electrodes,” Lee et al., Electrochimica Acta 43(24):3693–3701, 1998.
A more recently proposed solution is presented in U.S. Pat. No. 6,309,722 to Zuber et al. The '722 patent teaches adding insoluble components to the inks to induce porosity in the conducting polymer layer. The porosity partially overcomes problems associated with the catalyst being isolated from the reactants. However, the porosity does not overcome the problem of the catalyst being electrically isolated from the current collector or the problem of the limited proton conductivity of the cast proton-conducting polymer. In addition, achieving porosity adds time and cost to the preparation of the catalyst inks and layers.
Some prior art fuel cell applications seek a high density catalyst loading in order to achieve small fuel cell size. For example, in mini and microelectronics applications small fuel cells are desirable. For these applications, the prior art has had limited success in achieving suitably high loadings.
These and other problems in the art remain unresolved.